The effects of various precipitation conditions on the formation of silver halide emulsions comprising tabular crystals have been studied extensively as the said tabular grains are known in the photographic art for quite some time. As early as 1961 Berry et al. described the preparation and growth of tabular silver bromoiodide grains in Photographic Science and Engineering, Vol 5, No 6. A discussion of tabular grains appeared in Duffin, Photographic Emulsion Chemistry, Focal Press, 1966, p. 66-72. Early patent literature includes U.S. Pat. Nos. 4,063,951; 4,067,739; 4,150,994; 4,184,877 and 4,184,878. However the tabular grains described herein cannot be regarded as showing a high diameter to thickness ratio, commonly termed aspect ratio. In a number of U.S. applications filed in 1981 and issued in 1984 tabular grains with high aspect ratio and their advantages in photographic applications are described as, e.g., in U.S. Pat. Nos. 4,434,226; 4,439,520; 4,425,425; 4,425,426 and 4,433,048. A survey on high aspect ratio silver halide emulsions appeared in Research Disclosure, Vol 225, January 1983, Item 22534.
The above cited references on tabular grains are mainly concerned with high sensitive silver bromide or silver bromoiodide emulsions. Tabular grain emulsions having a high aspect ratio are known to provide several advantages over more conventional spherical grains as e.g. a high covering power, a high (spectral) sensitivity and a lower coating weight, which saves costs in manufacturing. Said lower coating weight is especially preferred if rapid processing applications are required, which is nowadays an ever more returning demand.
Spectrally sensitizing dyes are well known in the art of photography, especially for green and red sensitization of flat tabular grains, whereas for blue and/or ultraviolet sensitization the number of examples is rather limited. Further it is known to use in radiography combinations of green-emitting phosphor screens with film materials containing green sensitized tabular grain emulsions. After processing of exposed emulsion grains residual amounts of dyes may be present, especially due to the presence of huge amounts of spectral sensitizing dyes as tabular grains have a large specific surface capable of adsorbing said huge amounts. Those huge amounts are further in favour of high speed and image quality (especially sharpness), required in diagnostic imaging where it is further of utmost importance to reduce irradiation of the patient to minimum levels.
Specific layer arrangements developed in order to get the desired features mentioned hereinbefore have been described e.g. in JP-A's 08006192, 08076305 and 09197625; in EP-A's 0 661 592, 0 693 190 and 0 770 909 and in U.S. Pat. Nos. 4,585,729; 5,380,636; 5,718,994 and 5,460,916.
Especially in mammographic applications, wherein, for reasons of good image definition light-sensitive layers are present on only one side of the film support, and where image formation proceeds with a system consisting of only one intensifying screen, a high speed, a high contrast (preferably a high "toe contrast") and low residual dye stain are desired. Specific measures taken therefor have e.g. been described in U.S. Pat. No. 5,290,655; in EP-A's 0 264 788 and 0 577 027 and in Research Disclosure No. 33487 (1992), p. 161.